Triangular indexable inserts are used for turning and milling. The indexable inserts, in this case, have a main cutting edge and an adjoining secondary cutting edge, said cutting edges merging into each other in the region of one corner of the triangle. Such an indexable insert is known from U.S. Pat. No. 3,800,379. On its top surface along the longitudinal edges of the triangle, said cutting tip, which is used for turning, has main cutting edges and adjacent, short secondary cutting edges. A support surface, which has no cutting function whatsoever, is provided between the secondary cutting edge and the main cutting edge of the adjacent corner which lies on the same longitudinal edge. The bottom surface of said indexable insert is planar.
U.S. Pat. No. 5,876,160 discloses a triangular indexable insert where the corners of the triangle are approximately at 135° to the main cutting edge on the longitudinal edge. The corners, moreover, are formed by two butt-jointed secondary cutting edges which in each case extend in a straight line. Each secondary cutting edge is associated with a directly adjoining main cutting edge such that each main cutting edge has a secondary cutting edge adjoining it on a respective longitudinal end. Consequently, the indexable insert can be used as a left-handed and right-handed tip, i.e. with a left-handed and also right-handed spindle axis. The maneuverability of the cutting tip is produced by it being a negative indexable insert, i.e. its wedge angle in sections is 90°. Even in the case of the aforementioned U.S. Pat. No. 3,800,379 it is a so-called negative cutting tip which, however, cannot be turned but just rotated.